Ben Nelson, the conservative two-term Nebraska Democrat, won’t seek re-election to the Senate next year, according to Politico.

Nelson is 70–not exactly an adolescent, but hardly outside the norm for a Senate whose average member is 62. He had $3 million in the bank for what was expected to be a uphill re-election battle, with $1 million more from the Democratic Senatorial Campaign Committee. But he was also perhaps the most aggressively targeted incumbent on Capitol Hill, already facing a barrage of TV attack ads from outside groups, and he sported one of the single largest albatrosses in recent congressional campaign history.

The best thirty second explanation of his retirement is probably this ad from the Republican independent expenditure group Crossroads GPS, which has already dumped half a million dollars against Nelson:

His vote for the Affordable Care Act, allegedly secured by a special provision funding 100% of the Medicaid expansion in Nebraska–devastatingly dubbed the Cornhusker Kickback–seems to have done irrevocable damage to his standing in what was already a deeply conservative state.

Nelson made his retirement announcement in a video to supporters Tuesday afternoon, telling them that he wanted to spend more time with his family and look for other ways to serve. “Simply put,” he said, “it is time to move on.” That leaves Democrats in a tough spot.

State attorney general Jon Bruning and treasurer Don Stenberg are among the GOPers vying for the nomination to contest Nelson’s seat, but while DSCC chair Patty Murray focused on the “divisive primary” in her statement on Nelson’s retirement, it seems unlikely to change the outcome of the general. There’s not a deep bench of Democratic recruits in Nebraska and despite his blemishes, Nelson was Democrats’ best shot at holding the seat. His retirement makes the task of hanging onto the overall Senate majority that much trickier.